After an oil and gas well is drilled, tubulars are moved through a surface wellhead by a hydraulic workover rig. Tubulars are typically connected to each other by couplers to form a tubing string. The tubing string extends through a wellbore that is defined by equipment on the surface and by a well below the surface. The couplers define a larger outer diameter (OD) section of the tubing string as compared to other sections of the tubing string. Other components, such a downhole tool, can also be incorporated into the tubing string and, similar to the couplers, these other components can define a larger OD section of the tubing string.
A hydraulic workover rig typically uses a hydraulically-powered jack plate and slips to engage and move the tubular in the desired direction through the wellhead (i.e. into the well or out of the well). Tubulars that move through a wellhead must pass through one or more blowout-preventers (BOPs). One type of BOP is a ram BOP. A ram BOP has two, opposing hydraulically-actuated rams that move into a wellbore that is defined by the wellhead to form a seal about the outer surface of the tubulars. This seal contains the reservoir pressure of the well. However, different types of tubulars and even the same types of tubulars that may be moving through the wellhead can have different lengths. For example, one common form of tubular is referred to as pipe joint or a tubing joint. A tubing joint can have a length that ranges between about 7 meters and about 14 meters in length (one meter is equal to about 3.28 feet). Another common form of tubular is referred to as a pup joint. A pup joint can have a length that ranges between about 0.5 and 4 meters. This discrepancy in tubular lengths makes it difficult for an operator of the hydraulic workover rig to know when a larger OD section of the tubing string is approaching one of the ram BOPs.
A collision between any moving parts within a wellhead can be catastrophic for the well, the equipment at the well site and personnel in the area.